The National Hurricane Centre said the eyewall of the dangerous category four storm has reached the Texas coast, suggesting that the eye of the storm will make landfall in the coming hours.

The system is packing winds of 215 kilometres per hour, and experts fear could be the most destructive since Katrina left 1,800 people dead in 2005.

Texas Governor Greg Abbott has warned it could be “a very major disaster”, while US President Donald Trump made an early disaster declaration, unlocking federal funding.

Harvey is expected to generate storm surges along with prodigious amounts of rain. The resulting flooding, one expert said, could be “the depths of which we’ve never seen”.

Weather experts have warned areas of the state could be uninhabitable for weeks or months if Harvey is as bad as predicted.

Fuelled by warm Gulf of Mexico waters, Harvey grew from an unnamed storm to a life-threatening behemoth in just 56 hours, an incredibly fast intensification.

It is on track to make landfall at Rockport, a fishing-and-tourist town about 50 kilometres north-east of Corpus Christi. The National Hurricane Centre warned life-threatening storm surges could affect low-lying coastal areas.

“We know that we’ve got millions of people who are going to feel the impact of this storm,” spokesman and meteorologist Dennis Feltgen said.

“We really pray that people are listening to their emergency managers and get out of harm’s way.”

“We’re suggesting if people are going to stay here, mark their arm with a Sharpie pen with their name and Social Security number,” he said.

“We hate to talk about things like that.

“It’s not something we like to do but it’s the reality. People don’t listen.” Galveston-based storm surge expert Hal Needham of the private firm Marine Weather and Climate said forecasts indicated it was “becoming more and more likely that something really bad is going to happen”.

At least one researcher predicted heavy damage that would linger for months or longer.

It may also spawn tornadoes. Even after weakening, the system might spin out into the Gulf and regain strength before hitting Houston a second time as a tropical storm, forecasters said.

“In terms of economic impact, Harvey will probably be on par with Hurricane Katrina,” University of Miami senior hurricane researcher Brian McNoldy said.

“The Houston area and Corpus Christi are going to be a mess for a long time.”

While Mr Trump encouraged “everyone in the path of Hurricane Harvey to heed the advice and orders of their local and state officials”. The heavy rain could turn many communities into “essentially islands” and leave them isolated for days, said Melissa Munguia, deputy emergency management coordinator for Nueces County.

“Essentially there’s absolutely nowhere for the water to go,” she said.

Galveston Bay, where normal rain runs off to, will already be elevated.

The heavy rain is expected to extend into Louisiana, driven by counter-clockwise winds that could carry water from the Gulf of Mexico far inland. The Texas Governor activated about 700 members of the state National Guard ahead of Harvey making landfall.

Harvey would be the first significant hurricane to hit Texas since Ike in September 2008 brought winds of 177 kilometres per hour to the Galveston and Houston areas, inflicting $US22 billion in damage.

It would be the first big storm along the middle Texas coast since Hurricane Claudette in 2003 caused $US180 million in damage.